The invention is based on a ball joint for a brake booster. British Patent No. 1,511,672 discloses a ball joint, which comprises a first component in the form of an actuating rod with a ball head formed on it, a second component that is part of the brake booster and has an insertion opening with an annular groove, and a radially elastic securing ring, which can be snapped into the annular groove acting as a detent that keeps the ball head in the insertion opening. The ball head has a very complicated shape. To insert the securing ring into the insertion opening and displace it until it locks into place in the annular groove, a complicated assembly tool is needed. This makes automatic assembly difficult. Furthermore, this ball joint can be used only if it is still accessible to the assembly tool after the brake booster has been installed. In a ball joint disclosed by U.S. Pat. No. 3,733,966, the insertion opening is funnel-like near its beginning. This facilitates insertion of the securing ring. Once again, however, a special assembly tool is needed. This ball joint is again usable only if it is still accessible to an assembly tool after the brake booster has been installed in a vehicle. It accordingly became an object to provide an improved ball joint.